Field of the Invention
The present invention relates to a distributed index of refraction type optical element and a method of making the same.
Description of the Prior Art
As optical elements indispensable for optical systems to be developed in the next generation, distributed index of refraction type optical elements are now attracting wide attention due to their excellent ability for correcting aberration.
A large number of enterprises and research institutes are studying and developing various distributed index of refraction type optical elements, not to speak of SELFOC (registered trade mark) and Slab lenses which have already been offered for sale.
The distributed index of refraction type optical element has a refractive power of its medium proper which is imparted by the index of refraction distributed in the medium. The power is determined dependently on the distributed index of refraction and enhanced by increasing a difference between gradients of the indices of refraction (herein after referred to as .DELTA.n). Accordingly, an important task now imposed on researchers of distributed index of refraction type optical elements is to increase .DELTA.n and investigations are being made by many researchers to try to increase .DELTA.n.
Under the present circumstances, most approaches are made in the area of developing distributed index of refraction type optical elements having increased .DELTA.'s and large outside diameters, whereas attempts to reduce chromatic aberration in optical elements are retarded. Moreover, the distributed index of refraction type optical elements posses the possibility of remarkably reducing the number of lens elements owing to the excellent capability for correction of aberration, but such produces a contradiction in that the correction of chromatic aberration becomes more difficult as the number of lens elements is reduced. In order to compose a lens system comprising a distributed index of refraction type optical elements and having sufficiently corrected chromatic aberration, it may therefore be necessary to adopt a means such as use of an achromatic lens, thereby reducing the merit of the distributed index of refraction type optical elements. In order to compose a lens system in which the chromatic aberration is corrected with a small number of lens elements, it is impotant to reduce the chromatic aberration to be produced by each of the lens elements. For this purpose, the distributed index of refraction type optical element must be made of a medium having a characteristic ranging from the high index of refraction-low dispersion region to the low index of refraction-high dispersion region. In case of a radially distributed index of refraction type optical element, the index of refraction of the medium is varied and rays are refracted differently dependenting on height of rays (distance from the optical axis). Assuming now that a medium has a constant Abbe's number (.nu..sub.d =(n.sub.d -l)/(n.sub.F -n.sub.c)), the portion having a high index of refraction refracts a ray more largely than the portion having a low index of refraction, thereby allowing the ray to be dispersed more largely due to the difference in wavelength as illustrated in FIG. 1A. In other words, the chromatic aberration (n.sub.F -n.sub.c) becomes more remarkable as index of refraction n.sub.d is higher when Abbe's number is kept constant. In order to reduce the chromatic aberration (n.sub.F -n.sub.c ), it is therefore desirable that Abbe's number .nu..sub.d is large for the portion having the high index of refraction as shown in FIG. 1B. That is to say, it is desirable that a meduim has a charateristic varying from the high index of refraction-low dispersion region to the low index of refraction-high dispersion region. In case of the axially distributed index of refraction type optical element, on the other hand, it is considered that the conclusion obtained for the general achromatic cemented lens (doublet) shown in FIG. 2A or FIG. 2B is applicable. Since the cementing between the lens made of a medium having the high index of refraction and the lens made of a medium having the low index of refraction is realized by distributing index of refraction in a medium of a single lens as illustrated in FIG. 2C or FIG. 2D, it is desirable that the axially distributed index of refraction type optical element, like the radially distributed index of refraction type optical element, has a characteristic varing from the high index of refraction-low dispersion region to the low index of refraction-high dispersion region. Describing this characteristic with reference to the n.sub.d -.nu..sub.d graph illustrated in FIG. 3, the optical characteristic varying in the direction A is more excellent for correction of chromatic aberration than the characteristic varying in the direction B (See Japanese Patent Preliminary Publication No. Sho 60-218614). Further, as the distribution in the direction A in FIG. 3 has an indication closer to the horizontal direction, the chromatic aberration is not only reduced but also produced in the negative side. This is a phenomenon which cannot be observed in the ordinary glass material and an optical element exhibiting this phenomenon has an excellent capability to cancel the chromatic aberration produced by the other lenses. It is also possible, needless to say, to make an optical element which produces no chromatic aberration by properly setting the inclination.
However, most of the distributed index of refraction type optical elements now available are designed for increased .DELTA.n's and have smaller Abbe's number at the portion having higher indices of refraction. That is to say, most of the distributed index of refraction type optical elements have the characteristic varying in the direction B shown in FIG. 3. Speaking concretely of the ion-exchange method, for example, which imparts a gradient of density or concentration by ion-exchange between Tl.sup.+, a univalent ion adopted into glass as a component to compose a glass modifying oxide (having no direct relation to formation of glass) for increasing .DELTA.n, and Na.sup.+ or K.sup.+, the use of Tl.sup.+ allows an increase .DELTA.n but makes the Abbe's number have a characteristic varying from the high index of refraction-high dispersion region to the low index of refraction-low dispersion region, thereby allowing chromatic aberration to be produced remarkably. Further, ion-exchange between Ag.sup.+ and Na.sup.+ increases .DELTA.n but allows chromatic aberration to be produced remarkably. Furthermore, certain examples remarkably improve the correction of chromatic aberration by utilizing Li.sup.+ but decrease .DELTA.n on the other hand and do not exhibit the effects thereof sufficiently. Speaking more concretely, enhancement in content of Li.sup.+ is effective to increase .DELTA.n but have not attained to levels yet to exhibit sufficient effects in practice since enhancement in content of Li.sup.+ is allowable only within a limited range from the viewpoint of the chemical resistance of glass body materials and due to technical difficulty to dissolve a volatile alkali component stably into glass body materials. Since the ion-exchange method allows bivalent and higher-valent ions to be exchanged at very slow speeds and is substantially usable for imparting gradients of density of univalent cation only, the method allows the gradients of ion density, or concentration for imparting distributed indices of refraction to be varied only within a strictly limited range and does not permit obtaining a distributed index of refraction type optical element having a large .DELTA.n and producing little chromatic aberration. Moreover, distributed index of refraction type optical elements are being developed by the sol-gel method. There is a method to impart a gradient of density, or concentration by eluting a metal element such as Ti, Ge or Zr which enhances index of refraction and composes a glass forming oxide (a substance originally contained for forming glass) from a wet gel by using an acid or a similar chemical. Though this method permits obtaining .DELTA.n which is large to a certain extent, the distributed index of refraction type optical element prepared by this method has an Abbe's number characteristic varying from the high index of refraction-high dispersion region to the low index of refraction-low dispersion, and allows remarkable production of chromatic aberration, which is similar to the characteristic of the distributed index of refraction type optical element obtained by the Tl.sup.+ .revreaction.Na.sup.+ type ion-exchange.